The primary application of the present invention is envisioned to be in conjunction with the game of tennis. Tennis is a game in which the outcome of each point may be controlled by the determination of whether a ball lands within or on a given boundary, and is therefore "in", or whether the ball lands beyond the boundary, and is therefore "out". To the player of ordinary skill, this determination is generally within reasonable limits. The casual player's shots, and indeed the player himself, do not generally travel with great velocity. This ordinarily keeps the in/out determination within the bounds of human visual capabilities. Further, at the casual level, the in/out determination is usually not critical to matters other than personal pride. Generally, nothing more is at stake. However, as the skill level of the players rises, the situation becomes more complex.
Tennis officials have long recognized that at top levels, the speed at which the ball travels on shots, (in excess of 100 mph at times), combined with the fact that the player who would ordinarily be required to make the call will often also be moving at top speed and generally does not have the best visual perspective, frequently makes it impossible for the player to perceive accurately whether the ball was in or out. That the player does not have the best perspective of where a ball lands has been proven repeatedly. See, e.g., "In or Out", Robert H. Vincent, TENNIS, March, 1984, p.35; and "Vic Braden's Startling Revelations about Line Calls", Vic Braden, TENNIS, May, 1983, p. 37. Recognition of this fact has led to the use of linespeople at all major competitions. The linespeople are positioned so that they have a fixed parallel viewpoint of the boundary lines, which greatly reduces the number of erroneous calls. Unfortunately, the use of linespeople is prohibitively expensive for most play. Only those tournaments with large budgets can afford to use linespeople. Further, even with linespeople there is still the possibility of erroneous calls. Numerous studies, including those cited above, have consistently shown that the in/out determination is often quite simply beyond the bounds of human perception.
Improved accuracy in in/out determinations is desirable at all skill levels. However, at the top levels of competitive tennis, the skill and level of play differentials between the winner and loser of a given match can be very minute. One or two key points can determine the outcome. Considering that hundreds of thousands of dollars are at stake in the major professional tournaments, it is not difficult to understand the extreme desirability of eliminating human error in making line calls.
The prior art includes several examples of devices that have been built to provide a method of determining, on a tennis court, whether or not balls land in or out. The most common method involves a surface contact detection system in conjunction with a specially adapted tennis ball. See "Game Court Boundary Indicator System", Jokav and Grill. U.S. Pat. No. 3,774,194, issued Nov. 20, 1973; "Gaming face Contact Detecting Systems" Van Auken, U.S. Pat. No.4,1O9,9ll, issued Aug. 29 1978; and "Micro-Computer Network Systems for Making and Using Automatic Line-Call Decisions in Tennis", Supran, U.S. Pat. No. 4,432,058 issued Feb. 14, 1984.
The inherent disadvantage of these systems is of course that the tennis balls must be altered in one way or another. The Jokay device requires that three conductive winding be installed in the interior cavity of the tennis ball. The Van Auken system alters the exterior surface by requiring that at least one portion of the surface be electrically conductive. The Supran system also envisions the use of an electrically conductive ball. These conductive balls are likely to be prohibitively expensive. A further problem with altering the ball is that changing the ball may change the game. Thus a line calling system that requires modified balls could be expected to meet great resistance, as most players would be very hesitant to accept anything other than the standard tennis ball. Furthermore, the governing organizations and equipment manufacturers are highly resistant to change of this nature. The number of years it took for the optic yellow balls to be accepted is proof of the tendency.
A more useful sort of system is the "Tennis Court Line Monitoring Apparatus" of Grant, U.S. Pat. No. 3,982,759,issued Sept. 28, 1976. This system uses a series of small mechanical switches activated by the impact of the ball. This means the system will operate when a standard ball is used. However, since the switches are closed by physical contact, if a player is standing on the switched area, that senor cannot detect the impact of the ball. There is no way to detect the secondary impact. (Defining secondary impact as an impact on a switch that is already activated.)
In any system the detector area is broken into regions. In the switch type sensor, a region consists of multiple normally open switches all wired in parallel. With this arrangement any closed switch within a region closes the circuit for that region. Thus, if someone is standing in a region (activating it), the impact of a tennis ball anywhere within that region will have no effect (it will not be sensed). To minimize the risk of this occurring, a large number of smaller regions has to be used which requires a prohibitively large number of channels,
A second disadvantage of the switch type senor is that the means of discrimination between a footstep and a ball impact is limited to measuring pulse width. (It is inherently a single threshold device, with the threshold set by the mechanical parameters of the switch.) It can thus be fooled by a tap dancer, and likewise a quick footed tennis player.
Another method in the prior art is the use of light beams. One such system, known as "Cyclops", has been used at many professional tournaments. To date, the system has been used only on the service lines, not all the boundaries. One of the problems with this type of system is that anything that breaks the beam triggers the device. There is no way to distinguish between, for instance, a player's foot and the ball.
All the devices in the prior art suffer from at least one of the above shortcomings. They either require the use of a modified ball, they have no means to detect a secondary impact, or they have no means to differentiate between contacts caused by various objects.